Keeping track of printed documents where additional data, such as date of printing or copying, is steganographically stored within the printed document is an ongoing area of investigation. For a technique to be applicable to a greater number of applications, it is further desired that the technique is performed without affecting the visible quality of the original document along with the conflicting goal of being able to recover the additional data even from subsequent photocopies of the document.
Existing techniques which encode additional data onto printed matter superimpose a pattern of marks, typically dots, which contains the additional data, onto the printed matter. However, superimposing a pattern containing additional data over the entire original printed matter has many disadvantages. Some of these include substantial degradation of the quality of the document as well as difficulty in identifying dots within images due to little or no contrast between the dot and the region of the printed matter surrounding the location where the dot was placed. The situation worsens greatly when photocopies are made.
Other existing techniques encode additional data onto a page of printed matter selectively by identifying allowable encoding locations in the printed matter, usually the blank spaces near text, and place encoding dots at these locations. Although this avoids the problem of degradation of quality by encoding the additional data in selected locations on the printed matter, it will only work when there is sufficient white space in the printed matter.
Both modes of encoding additional data onto a page of printed matter have their limitations.